A scintillation material, or scintillator, produces light in response to incident radiation, typically ionizing radiation such as gamma rays or x-rays. Such a material can be a crystal such as thallium-doped sodium iodide, NaI(Tl), or a non-crystalline material such as a plastic.
Radiation detectors using scintillators include gamma cameras, x-ray detectors, and other radiation imaging or sensing devices. The scintillator generally has a radiation receiving face on which radiation such as gamma rays are incident. The scintillator has a light output face from which light emerges in response to incident radiation, and also an outside edge. For example, a disk-shaped scintillator would have a cylindrical edge surface. An optical window, such as glass, is typically bonded to the light output face of the scintillator. An array of light sensors, such as photomultiplier tubes, receive scintillation light emitted from the light output face of the scintillator, and transmitted through the window to the detectors. A plastic “light pipe” is sometimes interposed between the window and the light sensors.
The light is generated within the scintillator as pulses (also termed scintillation events). The positions of light pulses generated within the scintillator are used in imaging applications, such as SPECT (single photon emission tomography) and PET (positron emission tomography) imaging. Such uses may be generally referred to as scintillation imaging.
A problem in scintillation imaging is that reflections from the edge of the scintillator will reduce the measured positional accuracy of imaging data, particularly for scintillation events occurring close to the edge of the scintillator. Light from scintillation events occurring near an edge will be reflected from that edge, causing a serious reduction in ability to accurately determine the position of such events. In practice, a dead zone may exist around a peripheral region, proximate to the edge of the scintillator or window, from which meaningful positional data cannot be collected. This dead zone or “edge effect” reduces the effective usable portion of the scintillator. In addition, the presence of this unusable region at the periphery of the detector prevents the detector from being positioned optimally in many medical applications, for example breast and brain scintigraphy. The increase in detector bulk caused by the wasted portion of the detector may also make it difficult to position the detector sufficiently close to the patient in applications such as heart scintigraphy and many forms of SPECT.
In some detector designs, it is desirable to have more than one scintillator element in close proximity to another. In this situation, the junctions between elements tend to act as reflecting edges causing an unusable dead zone of edge effect on each side of the junctions.
This problem is well recognized in the field. For example, a previous attempt to solve this problem is described in U.S. patent application Publication 2003/0034455 to Schreiner et al., which suggests segmenting the scintillator into a number of triangular segments. However, such segmentation adds to the cost of a device, is difficult to fabricate and may cause problems if the scintillator absorbs moisture from the air. For example, it is well known that sodium iodide should be protected from atmospheric moisture.
Another attempted solution is described by U.S. Pat. No. 4,284,891 to Pergale et al., which suggests providing a diffused light reflector around the periphery of the optical window. However, it can be difficult to provide a true diffused reflector, as reflection properties of many materials and the crystal edge will change with time and environmental conditions. In addition, such diffused edge treatments have been found in practice to provide an unsatisfactory degree of improvement to the problem.
Hence, there is a need for improved scintillators and radiation detectors which reduce or eliminate the undesirable effects of edge reflection.